Automatic frequency control



June 24, 1941. R. J. KIRCHER AUTOMATIC FREQUENCY CONTROL Filed June 11,1940 3 Sheets-Sheet l BV I Ar Tom/,5v

June 24, 1941. R, 1 KlRCHER 2,246,688l

AUTOMATIC FREQUENCY CONTROL Filed June 11, 1940 3 Sheets-Sheet 2SELECTOR AND Mun vvvvvv IMI' I' ,UUUL N Q WML- 9 9 /m/ENTOR RJ K/RCHERJune 24, 1941.

R. J. KIRCHERy AUTOMATIC FREQUENCY CONTROL Filed June ll, 1940 5Sheets-Sheet 3 /A/VE/v TOR R J K/R CHE R m. .bfi

ATTORNEY Patented June Z4, 1941 AUTOMATIC FREQUENCY CONTROL Reymond J.Kircher, Neptune, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationJune 11, 1940, Serial No. 339,871

10 Claims. `(Cl. Z50-20) This invention relates to automatic frequencycontrol of the beating oscillator of a carrier wave receiving system.

An object of the invention is to maintain automatic control of the rstbeating oscillator of a radio receiver of the superheterodyne typeirrespective of fading or of the strength of the received incomingcarrier Wave.

An additional object of the invention is to prevent carrier wavereceiving systems having automatic frequency control from shifting so asto tune in an undesired radio transmitter during moments of fading ofthe desired carrier wave.

A further object of the invention is to prevent inordinate amplificationof noise and other extraneous oscillations during moments of .fading ofthe incoming carrier wave by reason of the fact that the automaticvolume control apparatus seeks to amplify the incoming carrier wave upto a predetermined level.

A still further object of the invention is-to provide a carrier wavereceiving apparatus with a local auxiliary or standby source ofoscillations which may be substituted for the incoming carrier wavesduring moments or intervals of severe fading or disappearance of theincoming carrier waves, and which auxiliary oscillations may be soautomatically controlled in frequency as to follow the frequency of theincoming carrier wave at all times.

Multiple detection stage systems for carrier wave receivers have longbeen well known. Such systems employ beating oscillators, theoscillations from which are caused to interact with the incoming carrierwave in devices commonly known as demodulators or frequency changers.Between the successive frequency changers the incoming energy is causedto traverse a band filter or other selective network which, in theinterests of noise reduction and selectivity, is ordinarily made to havea relatively narrow pass band sufficiently wide, however, to readilyaccommodate the essential frequencies of the signal band which it isdesired to transmit. Along with the fixed selective networks areemployed beating oscillators which are fixed with respect to frequencyto the highest possible degree by the use of piezoelectric controls in amanner which is well known. In order that such a system with its fixedselective networks may be employed in a receiver capable of receivingdifferent frequency carrier waves such as those arriving from differentbroadcast transmitters, it is necessary that the first beatingoscillator be made variable to permit the first intermediate frequencycarrier wave produced to fall within the band accepted by the firstintermediate frequency selective network. It will therefore be seen thatthe first beating oscillator which is also the highest frequencyoscillator of the radio receiver is necessarily much morer free'to varyin frequency than the other oscillators involved.

VMoreovery any slight change or drift in its frequency will result in acorresponding change in the intermediate frequency carrier wave which isproduced in the firstv frequency changer. It is therefore highlydesirable that the first oscillator be provided with anA automaticfrequency control as, for example, of one of the types disclosed in U.S. .Patent 2,041,855 granted May 26, 1936 toR. S. Ohl. However, sincesuch automatic frequency control devices are based upon energy derived`from the incoming carrier wave it may transpire that during severefading or momentary disappearance of the incoming carrier wave, theautomatic frequency control device may' no longer be effective andthereby allow lthe first oscillator to shift frequency or driftuncontrolled to such a point that upon recurrence of the incomingcarrier wave, the intermediate frequency waves produced'will not fallwithin the bands accepted by their respective selective circuits. Inaccordance with the present invention a source of auxiliary or standbyoscillations of substantially incoming carrier wave frequency isprovided and the source is connected to the input of the radio receiverto supply substitute oscillations for the incoming carrier waves onlyduring such intervals as the incomingcarrier waves experience severefading or disappear. In accordance with an additional feature of theinvention the auxiliary carrier waves are produced by combiningoscillations from the various local beating oscillators including therst beating oscillator, the frequency of which is automaticallycontrolled, thus insuring that at all timesv the auxiliaryoscillationsare in substantial frequency agreement with the incoming carrier wavesand are ready to take over the role of the incoming lcarrier waveswhenever the incoming carrier wanes below a useful magnitude.

Other features and objects of the invention will be apparent from aconsideration of the following detailed specication ltaken in connectionwith the drawings in which Fig. l illustrates by a line diagram oneembodiment of the invention in a carrier wave receiving system, Fig. 2shows a portion of the circuits of the system of Fig. 1, and Fig. 3 is aline diagram kof a modification of the carrier Wave receiving system ofFig. l. In the diagram the apparatus components which may be ofWell-known types are illustrated by labeled boxes. In the interest ofsimplicity the diagram indicates circuits by single lines, the directionin which the operation proceeds being shown by arrows.

Referring to the drawings an antenna or other receiving circuit I forincoming waves is associated with the input terminals of a radiofrequency amplifier R. F. A. to the output of which are connected intandem a first demodulator D1, a rst intermediate frequency selector and'amplifier, a second demodulator D2, a second intermediate frequencyselector and amplifier, a third demodulator D3, an audio frequencyamplier and finally a loudspeaker or telephone line. Connected to thedemodulators D1, D2, and Da are three beating oscillators B01, B02, andB03, respectively. In accordance with the Well-known superheterodynemode of operation, each demodulator serves to change the carrierfrequency of .the signal modulated Waves which yit receives by afrequency equal to that of its associated beating oscillator, th'eoscillator frequencies being so selected that the operation of the finaldemodulator yields currents of the modulation frequencies.

I Assuming that the incoming carrier wave is of 20 Ymegacycles frequencyandk is modulated by speech'signals, the antenna or receiving circuit. I

2| to the second intermediate frequency selector, the rst intermediatefrequency selector and the radio frequency amplifier respectively, asindicated. Moreover, the gain control may be extended to other elementsof the receiver, if desired. The gain control potential may be derivedfrom the unmodulated intermediate frequency carrier component selectedfrom the output of the demodulator D2. Y For this purpose a narrow-passfilter 'l may be employed With a transmission .characteristic Which issharply selective for frequencies Within a narrow range adjacent to thefrequency of the intermediate frequency carrier will preferably be madeselective to Waves of that frequency. Beating oscillator B01 may bedesignedto producek and to deliver toidemodulator D1 oscillations of afrequency of 17.1 megacycles.

The rst intermediate frequency selector and amplifier may be designed totransmit the intelligence band of 50: or `60 kilocycles in width inwhich occurs the speech-modulated intermediate frequency carrier wave of2900 kilocycles which resultsfrom the intermodulation of tl'ie'20megacycle incoming wave and the 17.1 megacycle local oscillations.Demodulator Dz receives the modulated'2900 kilocycle Wave and combinesit with oscillations of the frequency of 3000 kilocycles produced byoscillator B02 to yield signal modulatedV intermediate frequencyoscillations of a second intermediate frequency of 100 kilocycles Whichlare, in turn, impressed upon demodulator Da together With oscillationsof 100 kilocycles from the oscillator B03. The resultant currentsrepresenting the speech signal modulations are supplied by demodulatorD3 to the audio frequency amplifier and in turn to the loudspeaker ortelephone line.

In order to enable a receiving system of the type which has beendescribed to be tuned to different incoming carrier Waves at will and atthe same time to permit the various intermediate frequency selectors toremain fixed, it is desirable, andV infact customary, to make the firstlocal oscillator B01 of the tunable electric type. With the correcttuning of the oscillator B01 the intermediate frequency produced bydemodulator D1 will be precisely that which the remaining elements ofthe system are designed to receive. Moreover, the-local oscillators BOzand B03 may be made of the highly constant frequency type equipped withpiezoelectric frequency controls as indicated at 2 and 3.

In order tocompensate for fading and other undesired variations'in theamplitude of the incoming wave a gain ycontrol or automatic volumecontrol device 4 of Well-known type may apply gain control biaspotentials over paths 5, 6| and wave. kAs is Well known, the gain of thereceiver as a Whole may be controlled by such an automatic volumecontrol device in accordance With the amplitude of the intermediatefrequency carrier component. In the absence of excessive fading such adevice may be relied upon to maintain the intermediate carrier frequencyand consequently the signal-bearing sidebands at substantially aconstantamplitude level thus insuring that there is at all times anadequate volume of undistorted signal at the loudspeaker and that thevarious elements of the receiver including the loudspeaker are notoverloaded.

Inasmuch as the frequency of the ylocating oscillator B01 is notpiezoelectric controlled, there is a tendency for drifts in frequency ofthat oscillator to occur which may give rise to undesirable changes inthe intermediate frequency yielded by demodulator D1. To overcome this,oscillator B01 isprovided with an automatic frequency control which alsodepends upon the intermediate frequency carrier Wave selected by thefilter l. Con-` nected to the output `terminals of lter l is anintermediate frequency carrier amplifier 8 Vwhich employs theintermediate frequency carrier Wave and applies it to a discriminator`9, the output of which is connected to 'an automatic frequency controlI0 associated with the oscillator B01. For this purpose, discriminatorcircuits such as those described by Foster and Seeleyv at Fig. 4, page297 of the Proceedings of the Institute of Radio Engineers for March1937 may be employed. The

automatic frequency control and its connection to the rst'beatingoscillator may, for example, correspond to the circuit of Fig. 11 or tothat of Fig. 14 of the Foster and Seeley disclosure. As an alternative,the discriminator 9 and theY automatic frequency control l0 may bereplaced by the y.electromechanical synchronizing apparatus shown in thebroken line'box A of Fig. l of Ohl 2,041,855. In this case, the variablecondenser I9 of the Ohl disclosure Will correspond to a variable`capacity element of the beating oscillator B01. The apparatus 9 and l0Will serve to hold the frequency of the oscillator B01 in correctrelationship with the frequency of the incoming Wave so that thedifference frequency as evidenced bythe frequency of the oscillations inthe output circuit of demodulator D1 Will be substantially constant.

It may transpire inthe course of operation of a receiver such as hasbeen described that the incoming Wave will fade to such an extent thatthe intermediate frequency carrier component in the output'ofdemodulator D2 is no longer sufficiently strong tosufce for thefrequency control operation. `Under such circumstances, the gain controldevice will endeavor to increase the gain and will accentuate any noiseor other disturbance Which may tend to be accepted by the filter l. Itis possible that under these circumstances the automatic frequencycontrol may effectively lapse.

permitting the oscillator B01 to drift or change in frequency to such apoint as to enable some unwanted incoming carrier wave to secure controlof the receiver and to make it necessary to manually retune theoscillator B01 in order to receive the desired incoming wave when itreappears. In order to preclude such a faulty operation of the automaticfrequency control, this invention provides a local standby source ofoscillations which may be introduced into the receiving circuit I duringsuch periods as the intermediate frequency carrier wave falls below theamplitude desirable for maintaining automatic frequency control. Thelocal standby source comprises a combination of the three localoscillators B01, B02 and B03, together with combining devices in anarrangement such as to permit oscillations to be produced by rstcombining waves from two of the sources and then beating the resultantwith waves from a third source. In this manner, the standby oscillationsare of a frequency which, at every instant, is solely a function of thethree local oscillators. Consequently, if the three local oscillatorshave been operating at proper frequencies to receive a desired signalmodulated carrier wave, the frequency obtained from the standby sourceis of the proper numerical value so that if the incoming wave fades outthe standby oscillation may be supplied in lieu thereof to prevent driftof the system and particularly to prevent drift of the frequency ofoscillator B01.

The system for producing the standby oscillations comprises a combinerII to which .oscillators B02 and B03 are connected by paths I2 and I3.Assuming that oscillator B02 is producing oscillations of 3000kilocycles and oscillator B03 is producing oscillations of 100kilocycles, the combiner Il may be so designed as to select the lowerordifference frequencyfof 2900 kilocycles and to apply oscillations ofthat frequency to the path I4 which leads to a second combiner I5. Theoutput circuit of oscillator B01 may also be connected to the combinerI5 by means of a path I0. Combiner I5 may, therefore, produce and selectthe oscillations of the upper sideband or sum frequency of 17.1megacycles plus 2900 kilocycles or 20 megacycles. These standbyoscillations of 20 rnegacycles may then be applied over a path II to theattenuator and biased amplifier I3, the function of which is to preventtransmission of the standby oscillations when the intermediatefrequencyycarrier wave in the output of demodulator D2 is suflicientlystrong, but to permit the standby oscillations to be applied to theinput terminalsfof demodulator D1 over path I9 whenever the intermediatefrequency carrier waves in the output of demodulator D2 fall to a levelsuch that there is danger that the automatic frequency control will losecontrol of the oscillator B01. In Fig. 1, the device I8 is indicateddiagrammatically as including a normally open or inoperativetransmission path adapted to be closed or rendered operative under thecontrol of the automatic volume control electromotive force impressedover path 20. The device I8, which is illustrated in Fig. 2 and whichwill be explained in more detail in connection with the description ofthat figure, is of the electron dischargeampliiier type and is subjectedto the normal polarizing grid bias potential applied over the path 20.During normal operation when the desired incoming wave produces asufliciently strong intermediate frequency carrier in the output ofdemodulator D2 the automatic volume control device 4 maintains thebiased amplier I8 paralyzed and the standby oscillations are notimpressed from the source I5 upon the'circuit I. When, however, severefading reduces the intensity of the intermediate frequency oscillationsbelow a critical point automatic volume control device 4 removes theparalyzing grid bias from the amplifier I3 and permits the desiredstandby oscillation to be supplied to the rst demodulator. A mechanicaldevice such as a relay, or relays, operated by the automatic volumecontrol could also be used to perform the same function. The output pathI9 may in an alternative arrangement be connected to the circuit I tosupply the standby oscillations from source I5 to the input of the radiofrequency amplier. In the system, as illustrated, source B03 isconnected to the demodulator D3 by a switch 22. If desired, the switch22 may be thrown to its alternate position at 23 to disconnect thesource B03 from demodulator D3. In that alternate `position oscillationsof the intermediate carrier frequency present in the output ofdemodulator Dz, selected and substantially freed from accompanyingsideband components by the lter 1 may be impressed over the path 24after amplification by amplifier 8 upon the input of demodulator D3 tosupply the local beating oscillations for the final demodulation stage.*Fig 2 illustrates some of the circuit details of the system of Fig. 1including the attenuator and biased amplifier element I8. As indicated,the element I8 includes an electron discharge device of the pentode typeto the control grid circuit of which the output of combiner I5 isconnected to impress standby oscillations. The grid circuit is alsoconnected by path 20 to gain control device 4. The potential impressedover path 20 by the gain control 4 is normally such that the biaspotential produced across resistance element 25 in the grid circuit ofthe discharge device is sufficiently great to insure that the device beheld substantially in non-conducting condition. In other words, the netbias potential upon the control grid in consequence of the joint effectsof the potential across resistance 25 and the biasing potential ofsource 26 is sufficient to prevent passage of substantial space currentbetween the cathode and anode of the tube. The electromotive force ofsource 25 is less than the cut-off bias potential ofthe tube.Accordingly, when the current in path 2D decreases to tend to make thegrid less negative a point is reached at which the grid potentialpermits space current to flow in the discharge device. This point ispreferably made such that transmission through the element I3 beginswhen the intensity of the incoming carrierqwave has" so weakened as tomake continued control o f the oscillatorfrequency of the oscillator B01undependable. The tube of amplifier I8 will then begin to act as anamplifier and will transmit standby oscillations from combiner I5. Afteramplification by the discharge device these standby oscillations areimpressed upon an attenuator 21 from the output terminals of which they'are supplied at Aa low but reliable control volume to the input circuitof the rst demo-dulator D1 to take the place of the incoming carrierfrequency'oscillations which have virtually disappeared. The firstbeating oscillator B01 is connected over path 28 to the suppressor gridof the demodulator D1 and, accordingly', as a result of interaction ofthe standby oscillations supplied to the input of the demodulator D1 andof the local high frequency oscillations supplied by oscillator B01,intermediate frequency oscillations will be produced and supplied Vtothe first intermediate frequency selector and amplifier.

The system of Fig. 3 is similar to that of Fig. 1 but diifersprincipally in the apparatus for automatic frequency control of the rstbeating oscil-v lator. 'Ihe oscillator B01 is provided with a variablecapacitance element 29, a rotor element 30 of Whichis mechanicallyconnected to a shaft 3| driven vby a two-phase motor 32. The motor 32constitutes an element of an automatic frequency control motor unit 33which may correspond to the apparatus of the unit represented by thebroken line rectangle B of Fig. 1 of Patent 2,041,855 to Ohl. Such aunit must be provided with connections from two intermediate frequencysources one of which is dependent upon the incoming signals and theother of which is a 'local source independent of the incoming signals.'Ihe electrical connection which, in the Ohl disclosure, is made to theintermediate frequency oscillator 21 may, in the system ofFig.` 3, be'the path 34 leading from intermediate frequency oscillator B03. 'Iheconnection which', in the Ohl disclosure.' is made to the smoothingstage I3 or smoothing stage 20 may, in the system of Fig. 3, be the path35 from the intermediate frequency carrier amplifier 8. The amplifier 8is preferably a smoothing stage or, in other words, is a highlyoverloaded amplifier with a relatively sharply selective or narrowtransmission band output circuit. The operation of the device 33 willcorrespond to that of the apparatus B of the Ohl disclosure except thatwhereas in hl the motor serves to vary a capacity associated with anintermediate frequency oscillator, in the circuit of Fig. 3 the motorvaries a capacity element associated with the high frequency beatingoscillator B01, thus maintaining the tuning of that oscillator at theproper numerical frequency to assure efficient operation of thesuperheterodyne receiver with its subsequent xed intermediate frequencyselectors. In other respects, the apparatus and circuit features of thesystem of Fig. 3 correspond to the similarly designated elements of Fig.1.

What is claimed is:

1. A superheterodyne radio receiver for re` ceiving modulated incomingcarrier waves come prising a plurality of local sources of beatingoscillations for interacting with the incoming carrier wave wherebycurrents corresponding to its modulations may be derived and means re'-sponsive to the magnitude of the carrier component of the incoming wavefor introducing auxiliary locally produced oscillations of sub#stantially the incoming carrier frequency into the receiving system onlyduring such periods as the amplitude of the carrier component fallsbelow a predetermined minimum.

2. The combination accordingto claim 1, characterized in this, that thelocal sources of beating oscillations are interconnected Awithmodulators to produce the auxiliary locally producedoscillations ofsubstantially the incoming carrier frequency.

3. In combination, a circuit on which incoming carrier waves may beimpressed, a source of local oscillations for interacting with theincoming carrier waves, means for controlling the frequency of thesource of local oscillations to hold it xed with respect to thefrequency of the incoming oscillations, anV auxiliary source ofoscillations of substantially the incoming carrier wave frequency andmeans responsive to the magnitude of the incoming carrier Wave forconnecting the auxiliary source to said circuit only when the magnitudeof the incoming carrier wave falls below a predetermined value.

4. Atriple detection radio receiver comprising a receiving path, threelocal beating oscillators connected thereto to interact in successionwith received incoming carrier waves, an automatic volume control deviceconnected to the path andresponsive to the magnitude ofthe unmodulatedcarrier component of the received Waves, means for combiningoscillations from the three oscillators to produce auxiliary unmodulatedoscillations of substantially the incoming carrier wave frequency andmeans controlled by the automatic volume control device to apply theauxiliary unmodulated oscillations to the incoming terminal of thereceiving path when the unmodulatedcomponent of the incoming carrierwave falls below a preassigned limit.

.5. A heterodyne system for receiving carrier waves comprising a pathover which incoming waves may be received, a local source connected tothe path, means responsive to the variations in the frequency of thebeat frequency oscillations resulting from interaction of the incomingoscillations and those of the local source for oontrolling the frequencyof the local source, an auxiliary source for producing oscillationssimulating the incoming carrier waves in frequency and means responsiveto the magnitude of the beat frequency oscillations for applyingsimulated carrier waves from said auxiliary source toA said path onlywhen the magnitude of the incoming waves is less than a predeterminedlevel.

6. A carrier wave receiving system of the type in which the incomingcarrier wave is caused tol interact successively with oscillations froma plurality of local sources, the first of said sources -to interactbeing tunable to enable selection of a desired carrier wave and theremaining local sources being of a highly stable fixed frequency type,means for automatically controlling the frequency of the rst source tocause it to remain at a substantially constant frequency separation fromthat of the incoming carrier wave, means for producing auxiliaryoscillations of substantially the carrier frequency by combination ofoscillations from the local sources andy means responsive to themagnitude ofthe incoming carrier Waves for applying the auxiliaryoscillations to input terminals of the receiving system only when theincoming carrier wave fades below a predetermined level.

7. A multiple detection carrier wave receiver comprising a conductorover which incoming carrier waves may be received, a plurality offrequency-changing stages connected in tandem thereto, each of saidstages including a local oscillator, means for deriving oscillationsfrom each of the local oscillators and for combining them to produceauxiliary oscillations of sub'- stantially the incoming carrier wavefrequency and means responsive to the magnitude of the incoming carrierwave energy to apply the auxiliary oscillations to the conductor duringsuch periodsonly as the magnitude of the incoming carrier wave fallsbelow a predetermined minimum.

8. A successive detection receiving system for incoming carrer wavescomprising a conductor over which the waves may be received, a source oflocally produced oscillations for interacting with the incoming waves,an automatic volume control apparatus responsive to the magnitude ofresulting beat waves, a standby source of oscillations of substantiallythe incoming carrier wave frequency and means responsive to theautomatic volume control apparatus to connect the standby source to theconductor When the magnitude of the resultant beat waves falls to such apoint as to cause an inordinate amplication of noise.

9. A plural step detection carrier wave receiver comprising a conductorupon which received carrier waves may be impressed, a detector connectedthereto, a tunable beating oscillator connected to the detector toimpress locally produced oscillations thereon to interact with thereceived carrier waves and produce resultant waves of differentfrequency having the same modulations as the incoming carrier Waves, alocal auxiliary source of Waves of substantially the incoming carrierwave frequency and means con` trolled by the magnitude of the resultantWaves to connect the auxiliary source to the conductor to supplysubstitute waves for the incoming carrier waves during such periods onlyas said magnitude falls below a predetermined level.

10. A multiple detection carrier Wave receiving system of the typecomprising a plurality of local oscillation sources which interact insequence with incoming carrier waves, an automatic frequency controldevice for the first oscillation source responsive to resultant wavesproduced by interaction of the incoming carrier waves and oscillationsfrom the iirst source and means for preventing failure of the automaticfrequency control device when the incoming carrier waves fade out orotherwise drop below a definite level comprising an auxiliary source ofwaves to simulate incoming carrier Waves and a normally inactiveconnection from the auxiliary source to the receiving system which upondisappearance of the incoming carrier waves is rendered active to supplyauxiliary waves to the receiving system and thereby maintain operationof the automatic frequency control.

REYMOND J. KIRCHER.

